This invention relates to repeaters for PCM (pulse code modulation) or digital transmission systems, and more particularly to arrangements for compensating therein for transmission line characteristics.
Conventional automatic line build out (ALBO) networks provide variable equalization within a primary equalization feedback control loop to correct mainly for frequency dependent losses due to line length variations. However, changes in cable type, e.g., paper insulated, plastics insulated, etc., in cable characteristics and signal level variations all effect the performance of repeaters. The use of high efficiency line codes, such as that known as 4B3T and disclosed in U.S. Pat. No. 3,611,141, issued Oct. 5, 1971, requires accurate equalization of line losses, especially at lower frequencies where this code contains more energy than codes, such as that known as HDB3 (High Density Bipolar 3), whether or not the signals are scrambled.
A known equalizer circuit for heavily multiplexed PCM systems, e.g., 48 channels at 2.37 M bit/sec, for example, is shown in FIG. 1. This circuit incorporates an equalizer 13 including a shaping network SN, inductor L1 and a variable resistor R1; and a primary equalization control loop to keep the height of the so-called "eye diagram" or "eye pattern" constant, with the ALBO network 12 including shaping network SN1 and controllable impedance, such as the diode network 11, carefully designed to give correct equalization for a range of different line lengths. The equalized signal is taken from a decision point X and applied to an ALBO peak detector 10, which is a conventional known peak detector to detect the peak voltage or "height" of the eye diagram. This peak value is amplified and fed to a controllable impedance in the form of diode network 11, a controllable resistance. The output of diode network 11 is used to control the ALBO network equalization. When the repeaters are manufactured a manually adjustable variable resistor is included in shaping network SN1 to enable the repeater, once installed, to be adjusted for the low frequency content of the eye diagram.